The present invention relates generally to rotating electrical machinery such as motors and generators, and more particularly to the elimination or reduction of shaft voltages in an electric induction motor caused by common mode currents or by other conditions know in the art to produce shaft voltages.
Those skilled in the design and operation of rotating electrical machines have long recognized problems created by unwanted shaft voltages and bearing currents which can be induced as a consequence of routine operation of the machine. For example, conventional alternating current (AC) induction motors are often driven by a variable frequency inverter drive. High frequency signals from the inverter can create an electric field between the stator and the rotor of the motor. The electrostatic field and capacitive coupling between the stator and rotor can produce common mode currents, thereby producing voltage on the shaft of the motor. If the shaft is not grounded, the induced shaft voltage can result in an unwanted current flow through a path created by the shaft, bearings, and motor frame. Even though the shaft voltages may be low, the relatively low impedance of the bearings and current path can produce current of relatively high magnitude.
Frequently associated with steady or intermittent bearing currents is arcing across the moving parts of the bearing, from race to ball to race. Over time, this arcing can produce pitting of the metal surfaces and possible bearing failure and, ultimately, motor failure.
Several solutions to the bearing current problem have been proposed or used in the prior art, with limited success. One method that can be used is to insulate both bearings of a two bearing motor. However, this creates unwanted additional expense in the manufacture of the motor. Moreover, if the motor shaft is connected to another load with a bearing, the shaft voltage will be transferred to that load bearing.
Others have attempted to eliminate motor bearing currents by grounding the shaft using slip rings and brushes mounted to the shaft. Unfortunately, this solution has generally been found to be unacceptable in some applications for several reasons. The use of slip rings and brushes increases the cost of the motor assembly significantly and they are difficult to mount. The brushes also create dust contamination in and around the motor and tend to wear out. This adds maintenance time and expense.
An analogous solution that has been used in motors for computer disc drives has been electrically conductive grease ("ECG") to dissipate the electrical charge that accumulates in the motor bearings from static electricity. In U.S. Pat. No. 5,454,724 (the '724 patent), a method and apparatus for providing a controlled resistance electrical path from the rotor to stator in an electric motor used in a disc drive. ECG is applied between the rotating and static element of an electric drive. The ECG then allows the flow of electrostatic charge where it maintains a reliable current path between rotor and stator via the housing. This solved the problem of the build-up of an electrostatic charge between the read-write head of the drive and the disc itself. The problem remains in that the '724 patent requires a pre-loaded ball-like sphere immersed in ECG to be mounted and held in a center hole in the end of the shaft of the motor. Therefore, the problem of unwanted bearing currents arising in general electric motors still remains because the '724 patent does not teach a method for that is applicable to all conventional motors or a method that can be retrofitted to any motor.
What is needed, then, and not found in the prior art, is an inexpensive, durable, and convenient way for eliminating or reducing shaft voltages caused by common mode currents in electric motors driven by variable frequency inverter drives, or by other factors.